Continuous phase modulation (CPM) is a widely used modulation format. For example, the global system for mobile communications (GSM) uses the gaussian minimum shift-keyed (GMSK) modulation format that is based upon frequency, phase, gain/amplitude, and timing recovery feedback loops to achieve a theoretically optimal bit-error rate (BER) performance. Although computationally intensive, a CPM signal may be demodulated with a non-coherent decoder, and may still match the optimal coherent bit error rate. Ultra high frequency (UHF) satellite communications (UHF follow-on (UFO) SATCOM) may severely distort the transmitted signal, thus causing increased loss in power efficiency or bit error rate performance.
Non-coherent decoding of CPM signals typically requires correlation banks that are matched to the expected I&Q signal values over several symbol periods. The received signal is compared to each possible valid combination of symbols across multiple symbol periods. The best correlation may indicate the most likely symbol transmitted. U.S. Pat. No. 7,636,399 to Brown et al. discloses a non-coherent receiver comprising a bank of CPM waveform matched filters for obtaining branch metrics for each consecutive CPM symbol. The device also includes a recursive inner decoder cooperating with the bank of filters.
Non-coherent CPM decoding is based upon the phase being continuous throughout the multi-symbol correlation process. This may not match the definition of traditional “non-coherent” demodulation. Thus, another more appropriate term may be “quasi-coherent.” This multi-symbol correlation process may not be based upon the receiver recovering the transmitted phase of the incoming signal, as is typically performed with a phase-lock loop (PLL). A receiver that generates a copy of the transmitted carrier frequency and phase is typically defined as a coherent receiver.
Some communications modes use a burst/packet communications waveform and operate in a harsh interference environment, for example, for communications over a satellite channel. Due to the nature of these communications modes, a coherent receiver may be increasingly difficult to design, as it is typically difficult to recover the transmit carrier frequency and phase.
Accordingly, a receiver that operates in such modes may use the quasi-coherent CPM demodulation technique. However, the distortion and intersymbol-interference induced by a satellite channel, for example, a 5 kHz UHF satellite channel, generally causes an irreducible bit error rate (1E-4 for a FSK receiver, 1E-5 for a non-coherent CPM receiver).
A standard for UHF satellite communications (Integrated Waveform) includes CPM modems that may be used on UHF satellite transponders. IW supersedes Demand-Assigned Multiple Access” (DAMA) MIL-STD-188-182 (A), MIL-STD-188-183 (A) military interoperabilty standards for UHF Follow-On (UFO) Satellite Communications. IW is described in MIL-STD-188-181C, MIL-STD-188-182B, and MIL-STD-188-183B interoperability standards. There are two transponder types that are designed to support two channel bandwidths—25 kHz and 5 kHz.
To meet the bit error rate requirements, terminals or receivers may use least mean square (LMS) equalization. LMS equalization uses an error calculation that is based on the difference between the expected received signal and the actual received signal. If the expected received signal that is generated for this difference does not account for the carrier frequency and phase, or the actual received signal is not modified to remove the carrier frequency and phase offset, the LMS receiver attempts to remove the carrier frequency and phase because it will see it as an error.
Unfortunately, for UFO satellite communications, the LMS linear equalizer cannot simultaneously adequately address both the inter-symbol interference (ISI distortion) caused by the satellite channel and the carrier frequency and phase offset. An LMS equalizer will attempt to adapt to any channel impairment, with some loss due to the inability of a linear filter to accurately model a satellite channel with both linear and non-linear impairments. An LMS equalizer will also attempt to remove phase, frequency, timing, and any other error. However, it is increasingly difficult to measure error in an LMS equalizer.
Error is defined as any difference (typically complex-valued in-phase and quadrature) between the received signal and the expected/desired signal. Thus, a received signal with a frequency offset may appear as an error to the least-mean-squares error calculation and the linear filter that is at the receiver to be used to approximate the channel. This linear filter typically has coefficients that are modified using the LMS coefficient calculation to remove the frequency error. Typically, frequency, phase, and timing errors are not expected, but rather they are known ahead of time.
U.S. Pat. No. 7,088,793 to Mickelson et al. discloses an equalizer for use with complex modulation modems to reduce inter-symbol interference. The equalizer includes an equalizer filter that receives an input data signal and adapts the input signal to compensate for the noisy communications channels to reduce inter-symbol interference. A branch metric computer demodulates the equalizer filter adapted input data signal.